Find Proof of Mae'Var's Treachery is a side quest in Shadows of Amn companion which potentially leads to Thieves' Guild Stronghold quest
In Chapter 2, go to the Shadow Thief Guildhall located in northeast of the Dock area, speak to Renal Bloodscalp on the second floor, who will ask you to infiltrate Mae'Var gang of thieves, which is a part of the Shadow Thieves. Renal needs prove that Mae'Var is plotting against him. Do whatever task is required to earn his trust. He will give you a Letter of Transfer.
Down the stairs in southeast party of Dock, enter Mae'Var's Guildhall and show the letter to Gorch behind the counter. He will grant access to the guildhall. Go through the door and then down the stairs beside you. Behind a secret door, you will find Mae'Var torturing a thief named Lin. He will give you your first assignment, depending on your Reputation.
If your reputation is 10 or higher, you will be given the task to steal from the Temple of Talos. If your reputation is 9 or lower, you will have to steal from the Temple of Lathander. Make sure it is nightfall before you do either of the two tasks.
Enter the Temple of Talos in the Athkatla Temple. Pick the lock of the chest in Weathermistriss Ada's chamber and take the Necklace of Talos. Give the necklace to Mae'Var in his Guildhall and he will tell you to report to Edwin, his second man in command.
Go to the Temple District. The entrance to the Temple of Lathander is north in this area. Enter the Temple of Lathander. The Statue of Lathander is in a strongbox located in the temple. However, it is guarded by Dawnmaster Kreel is hanging around. At day there are also two Morning Knights. You can't burgle the temple's strongbox so long as anybody is there. If you do, all NPC's in the temple will turn hostile towards you.
Have your party Thief hide in the shadows, and have everybody else leave the area. Wait for Dawnmaster Kreel to go away. When he is out of sight, pick the lock on the strongbox and take the Statue of Lathander. Go back to Mae'Var and deliver the statue and he will tell you to report to Edwin, his second man in command.
Get to the third floor of the guildhall and speak to Edwin, a former companion in Baldur's Gate. He will require that you kill a Cowled Wizard named Rayic Gethras for him, who lives in the docks district. Enter Rayic Gethras' house to the west and defeat the wizard on the third floor. Return to Edwin to report your success. Next he will ask you to retrieve some documents from a merchant named Marcus.
Enter the Sea Bounty Tavern just east of the guildhall. You have many ways to deal with Marcus, pay him, threaten him if your strength is 16 or higher, kill him silently if you're a thief, or try to pickpocket the documents from him without even needing to speak to him, though you need a decent pickpocketing score for this. If you're a female with 12 charisma or higher, he may just give them to you with some flattery.
Give the documents to Edwin and he will tell you to report to Mae'Var for your final assignment.
Return to the cellars of Mae'Var's Guildhall and speak with Mae'Var. He will require that you kill a traitor named Embarl. Enter the second floor of the Sea Bounty and speak to Embarl. He overheard Mae'Var say that he wants to kill Renal Bloodscalp but he has no proof. Ask him to give you his dagger and let him leave. Give the dagger to Mae'Var as proof that you killed Embarl.
Speak to Edwin once more and he will reveal that he wishes to betray Mae'Var. He will give you Mae'Var's Strongbox Key and he will offer to join your party. Go down a floor and use the key to open the strongbox in the north chamber. Take Mae'Var's Letter from the box
Give the letter to Renal Bloodscalp in the Shadow Thief Guildhall. He will thank you for proving Mae'Var's guilt and will ask you to return to Mae'Var and kill him. Return once again to Mae'Var's Guildhall and fight your way through to Mae'var in the cellar. Once you have killed him, return to Renal Bloodscalp for a handsome reward.
Remember to loot Mae'Var's place clean and do not store any important items there before
reporting back to Renal for rewards, for you have no means to return to original Mae'Var's
Guildhall after accepting it as your stronghold.